1. Field of the Invention
The present invention relates to an overhead cable such as an overhead electric cable and an overhead earth cable and, more particularly, to an overhead cable which is less subject to a wind load under conditions of strong wind in a typhoon or the like or coexistence of strong wind and heavy rain, and which furthermore makes less wind noise at a medium wind speed.
2. Description of the Related Art
Conventionally, as an overhead cable in which a wind load is more reduced than in an aluminum conductor steel reinforced (ACSR) in which round element wires are stranded, an overhead cable in which spiral grooves are formed on the outer circumferential surface is known to the public (Japanese Patent No. 2898903, and Japanese Patent No. 3540720).
However, although these cables can reduce a wind load at the time of strong wind, these cables make large wind noise when wind having a wind speed of 10 to 20 m/s blows, and hence these cables are not suitable as overhead power transmission cables passing near private houses.
In order to reduce wind noise, it is effective to provide spiral projections on the overhead cable. According to the result of wind noise measurement carried out by means of wind tunnel facilities by using a cable described in Japanese Patent No. 3540720 on which spiral projections are provided, it was found that an effect of wind noise reduction cannot be obtained unless the size of the projection is made large because of the influence of the grooves formed on the surface of the cable. However, if the size of the spiral projections is increased, the drag coefficient becomes large and, as a result, the precious wind load reduction effect is deteriorated.
As described above, reduction of the wind load and reduction of the wind noise are in a conflicting relationship, and it has been difficult to make them compatible with each other.